This is a continuation in part of application Ser. No. 668,307, filed Nov. 5, 1984 which is incorporated herein by reference.
This invention relates to a hydraulic lifting mechanism for an article of furniture such as a chair wherein a hydraulic power unit is provided for raising and lowering a retractable support member for supporting a chair seat or the like and which includes a reservoir containing a two phase working fluid.
It is desirable in certain articles of furniture to provide a mechanism for adjusting the height of a supporting member such as for example the height of a chair seat so that a chair occupant can select the seat height best suited for him. Prior art hydraulic mechanisms have been provided for this purpose and those mechanisms have in general included a pair of rigid walled hydraulic fluid filled chambers which communicate with each other by means of a hollow conduit. One of the chambers is expandable so as to have a variable volume and the other chamber contains a compressed gas in addition to the hydraulic liquid. The hydraulic fluid may be transferred from the pressurized chamber to the expandable chamber when a control valve located in the hollow conduit is opened. As the volume of fluid contained in the expandable chamber increases, the support member will be urged upwardly, thereby raising the chair seat. The chair seat is returned to its retracted position by transferring fluid from the variable volume chamber to the pressurized fixed volume chamber. As the variable volume chamber is compressed and the fluid is forced out of that chamber and into the pressurized fixed volume chamber, the volume of hydraulic liquid in that chamber is reduced.
One prior art patent showing the structure hereinabove described is U.S. Pat. No. 4,074,887. This patent shows two circumferentially arranged chambers with the outer chamber comprising a rigid walled sealed chamber containing hydraulic fluid and containing a pressurized gas in a top portion thereof. The inner chamber is a rigid walled expandable chamber containing only hydraulic fluid. A control lever is provided for operating a valve to enable hydraulic fluid to be transferred by means of the gas pressure from the outer chamber to the inner chamber. A piston in the expandable chamber is raised and lowered by operation of the control valve and transfer of the hydraulic fluid between the chambers.
A disadvantage of the prior art structures is that sliding seals are needed to seal the piston. Such seals are subject to wear and leakage and, as hydraulic fluid leaks out of the mechanism, the volume of hydraulic fluid in the chambers is reduced. The pressurized gas in the outer chamber will then occupy a greater volume, thereby reducing its pressure and causing the upward force and speed of the chair seat to be reduced which is undesirable. Furthermore, the lost hydraulic fluid needs to be replaced from time to time thereby requiring servicing of the mechanism. It is therefore desired to provide a hydraulic lifting mechanism wherein no sliding seals are needed and which is not subject to leakage and loss of hydraulic fluid.
Another disadvantage of the above described prior art mechanisms is that, as hydraulic liquid is transferred from the fixed volume chamber to the expandable chamber, the upward force on the chair seat and the speed thereof will be reduced. The reason for this is of course that, as the gas volume increases, the pressure decreases. It is therefore desired to provide a hydraulic lifting mechanism wherein the gas pressure is relatively constant so that the upward force on the chair seat and the speed of travel of the chair seat remain constant throughout the adjustment range of the chair seat.
A further disadvantage of the prior art mechanisms has been that the size of the fixed volume chamber needs to be relatively large in order to be able to contain a sufficient amount of pressurized gas to cause the hydraulic the variable volume chamber. It is therefore desired to provide a hydraulic mechanism for performing the lifting functions of a support surface in an article of furniture wherein the size of the fixed volume chamber is relatively small.
A still further disadvantage of the prior art structures has been that the controls for adjusting the lifting mechanisms have been located adjacent to the mechanism. These prior art arrangements have necessitated the occupant of the chair to lean over and reach far under the chair seat or have necessitated long control linkages for operating the control valves. It is therefore desired to provide a hydraulic lifting mechanism wherein the control valve can be located in a convenient location in either the chair arm or adjacent the chair seat so that it is easily operable by a chair occupant.
A yet further disadvantage of the prior art structures has been the provision of complicated valves to control the transfer of hydraulic fluid between the chambers. It is therefore desired to provide a hydraulic mechanism including a control valve which is simple yet reliable and effective.
In some of the prior art lifting mechanisms only a gas rather than an incompressible hydraulic liquid has been used to provide the lifting functions of the mechanism. These types of mechanisms are subject to leakage of gas from the mechanism and also result in spongy action of the mechanism since the gas is compressible. It is desired to provide a very positive hydraulic lifting mechanism wherein the hydraulic working fluid is incompressible.
In still other prior art mechanisms manual or electric pumps have been provided to pump the hydraulic fluid to the variable volume chamber to cause the lifting action. Such pumps are subject to failure and are also costly. It is therefore desired to provide a mechanism wherein no pumps are needed to transfer hydraulic fluid between the two chambers.